This invention relates generally to locating underground objects, and more particularly the invention relates to locating an underground object in the presence of other underground objects.
Buried pipes and conduits are used by a large number of utility companies including telephone, water, gas, CATV, and electric power. Frequently it becomes necessary to excavate in an area where one or more of these conduits have been placed. It also becomes necessary to repair broken or leaky conduits. Conversely, it is important to know the location of each of the conduits or pipes before any digging or excavation begins.
In order to facilitate the replacement and repair of buried utility systems it is important to have a simple and quick method of locating the true position of the buried conduits. Most buried utility systems are constructed of materials which produce a magnetic field when electric current is passed through them. In cases where the pipe or conduit is made of a non-conductive or non-magnetic material, there is typically a wire or metal line placed in direct proximity therewith. AC currents can be induced in the conductive wire or pipe thus producing an AC magnetic field.
There are a number of existing devices which are used to locate buried or otherwise concealed electrically conductive objects. The devices use an alternating current which is impressed on the concealed conductive object, such as a pipe or cable, by direct connection or by inductive coupling. The object can be located in a horizontal underground plane or in a lateral plane by use of a suitable horizontal axis pickup coil and amplifier with an appropriate indicating device such as a meter or audio transducer. When the receiver pickup coil is brought closer to the object being investigated, the AC signal level increases and the position nearest the object produces the strongest signal.
Several instruments have been developed to provide this function. For example, U.S. Pat. No. 4,387,340 issued to E. Peterman discloses a receiver having four sensors mounted in a fixed relationship. Two of the sensors (antenna coils) are used for left-right determination of the position of the buried conductor. In operation, the axes of both coils are horizontally disposed and the coils are horizontally spaced. The outputs are subtractively combined to produce a signal which is related to the difference between the magnetic flux in each coil. Thus when the device is centered over the conductor, the field strengths through the coils cancel each other out, which is reflected in an appropriate display device. When the difference is non-zero, the phase of one horizontal coil is compared to the phase of the other to determine left-right position.
A device operating on a slightly different principle is described in U.S. Pat. No. 4,427,942 issued to L. Sole. That device employs two coils whose axes are horizontally disposed, the coils being separated vertically. The coil signals are rectified and the average DC voltages compared. When the apparatus is directly over the conductor, the two signals are essentially equal (non zero), and an indicator alerts the user to this fact. This device does not, however, provide left-right guidance.
Another invention operates on a phase comparison principle similar to Peterman U.S. Pat. No. 4,639,674 depicts an apparatus having three coils, all vertically disposed, lying in a triangular formation, the plane formed thereby being vertical also. A flip-flop serves as a phase detector between the two signals from the lower coils. The third cell acts to eliminate of ambient magnetic fields.
Three other patents of interest employ phase comparison to locate the conductor. U.S. Pat. Nos. 4,134,061 issued to H. Gudgel; 4,438,389 issued to A. DeSa; and 4,390,836 issued to Bruce et al. Each includes additional coils for locating faults in the conductor in addition to tracing the conductor path. The '836 patent employs two horizontally disposed coils for tracing the conductor path.
The use of orthogonally disposed coils is also illustrated in U.S. Pat. No. 4,220,913 issued to Howell et al. Four coils are shown in a rectangular arrangement, however, only the two vertically disposed coils are initially used to determine whether the unit is directly above the buried conductor. Left-right determination is then made by comparing the phase of the signal from one of the vertical coils to the phase of the signal from one of the horizontal coils. In other words, it is not necessary to use all four coils for left-right determination (the "extra" coil is used for depth determination).
Another orthogonal configuration of coils is disclosed in U.S. Pat. No. 4,542,344 issued to Darilek et al which uses two sensors horizontally separated at a distance R, which are held a fixed distance from the ground by a rod. Each sensor is comprised of two coils, one vertically disposed and one horizontally disposed, resulting in four leads. The leads are multiplexed and combined in to a single output fed in to an Automatic Gain Control (AGC). The AGC averages the amplitude of each signal against the prior four signal amplitudes, this produces a weighted value for each coil. These value are used to calculate the left-right offset.
A problem with these systems arises when several pipes are located in the same area and the location of all pipes is desired. Signals transmitted by several pipes can interfere and complicate the detection process. U.S. Pat. No. 5,264,795 to Rider includes an encoded digital or analog code in the carrier in order to facilitate identification of a transmitter and conduit. The transmitter uses a discrete frequency and includes a transmitter identification code sent as a modulated signal. Other information can be modulated on the carrier including voice and tones to reveal battery conditions.
The present invention provides a method of locating one or more underground objects utilizing concurrent radio wave transmission by employing spread spectrum modulation techniques.